1) Shares were massively overvalued compared to their yields. E.g. if P/E ratios were 50:1 then I wouldn't buy the market
2) There's a fundamental issue in the economy. Plague, war, asteroid, climate change. Whatever the issue, you probably have bigger things to worry about than your retirement fund, and you have no better bets to place
The stock market is just made up of businesses. Businesses make money. Owning part of a business gives you a return. There's no magic to it, no massive extrapolation based solely on past performance. If you think there's a decent chance businesses aren't making money, then you ought to become a prepper.
Markets can grind lower and lower for decades, each new low triggering bargain hunters to buy, only to drop more.